The present invention relates to a method for controlling an internal combustion engine and more particularly to a method for adjusting injected fuel based on a prediction of air entering a cylinder for future induction events.
Determining an engine air amount for individual cylinder induction events is important to properly fuel an engine. Typically, an engine air amount is calculated prior to fueling, as many as two engine events prior to an intake event. This is important because fuel is usually delivered before an intake valve opens so that fuel vaporization is promoted and emissions are reduced. Also, accurate engine air amount estimation is especially important during starting and run-up when exhaust gas after-treatment systems are not operating at optimal efficiency. Catalysts require elevated temperatures to operate efficiently. Catalyst temperatures rise as a result of engine operation, but are relatively low during start necessitating accuracy in engine air amount calculations and fuel delivery.
One method to predict an engine air amount is based on monitoring changes in throttle position, as disclosed in U.S. Pat. No. 6,170,475 owned by the assignee of the present invention. This method utilizes a throttle model that characterizes throttle flow given a throttle position and the pressure drop across the throttle. The model is described in look-up functions and tables that capture the physical behavior of the system. Prediction of an engine air amount is accomplished by sensing current and previous throttle positions, determining the relative rate of change in throttle position, then extending this rate of change so that a future throttle position is predicted. The predicted throttle position is then input into the throttle model to predict future engine air amount.
The inventors herein have recognized that this prediction method is not as accurate while the throttle position is not changing. Since a change in throttle position is necessary to predict a change in an engine air amount in the before-mentioned method, the method does not predict a change in an engine air amount during starting.
Another method to predict an engine air amount is based on a Mass Air Flow (MAF) sensor, as disclosed in U.S. Pat. No. 5,331,936 owned by the assignee of the present invention. This method describes using a MAF sensor in series with a throttle body and an intake manifold. The MAF sensor signal is ignored during start while the sensor signal is not ready, because the sensor element requires time to warm-up. The MAF sensor signal is enabled, in a specified time representing sensor warm-up time. After the MAF signal is enabled, usually during engine run-up, a model is used to predict future engine air amount.
The inventors herein have also recognized that while this approach works well during normal engine operation, it is not as accurate during start because the sensor is not warm and operational. During start a predetermined engine air amount is used in place of a measurement. Therefore, a constant engine air amount is provided while the intake manifold is being pumped down even though the actual engine air amount is changing.
In accordance with the present invention a method that accurately predicts an engine air amount during start is presented.
The method comprises: calculating an engine air amount based on at least a change in engine speed; and adjusting fuel supplied to the engine at least during an engine start based on said engine air amount calculation. This method can be used to reduce the above-mentioned limitations of the prior art approaches.
By using a change in engine speed to predict engine air amount, then adjusting fuel supplied to the engine for future cylinder events, the inventors herein have improved the prediction of engine air amount during a start. Since a change in engine speed can have a large effect on engine air amount during a start, the correlation between the two variables can be used to predict future engine air amounts. When a change in engine speed is used, an engine air amount may be calculated without limitations imposed by throttle prediction or MAF sensor characteristics. Also, a change in engine speed is readily calculated during start, run-up, and normal engine operation.
In other words, a change in engine speed produces a change in an engine air amount because the dynamics of pulling air into a cylinder are changing as the engine accelerates. Volumetric efficiency and gas kinetics change with a change in engine speed, producing a change in engine air amount. This relationship between a change in engine speed and a change in engine air amount has allowed the inventors to predict an engine air amount based on a change in engine speed.
By identifying the relationship between change in engine speed and predicted engine air amount the inventors herein recognize many possible configurations. Various examples may use variations of a change in engine speed including: difference in speed (xcex94N), difference in speed over change in time (xcex94N/xcex94time), xcex94N processed through a transfer function or difference equation, using current and past values of engine speed, using engine speed from current and past engine related events, interrupt driven speed measurement, a processed change in engine position, a processed change in engine position/change in time, using a processed engine position at current and past events, and interrupt driven processed engine position measurement.
The present invention provides the advantage of improved air/fuel control during start, resulting in lower emissions. This advantage is especially important when a catalyst is cold and its efficiency is low.